Mdt 6 Crack: Why You Should Upgrade to MDT 6 for Better Performance and Compatibility

Following this verification step, a zirconia restoration was fabricated and delivered (figure 4). While screwing the restoration into place, a cracking sound was heard. The restoration had fractured lingually to the access hole of dental implant No. 27 (figure 5).

Mdt 6 Crack

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A discrepancy between the model and the mouth must have been present, which caused the zirconia to stress and crack where it did. It is difficult to understand how this occurred, however, since a rigid resin impression and a verification jig were used in this case.

When the lab technician looked back at the original resin pattern, he noticed a fracture in the resin in the exact location (figure 6) where the crack had occurred in the full-arch restoration. Because the resin was too thin in that particular spot, it must have cracked slightly either when the impression was pulled from the mouth or when it was attached to implant replicas and poured into stone.

Electromagnetic methods such as eddy current, magnetic particle or radiographic and ultrasonic methods all introduce electromagnetic or sound waves into the inspected material in order to extract its properties. Penetrant liquid techniques can detect cracks in the test material by using either fluorescent or non-fluorescent dyes. In addition to these methods, scientists such as Shujuan et al. [2], Noorian et al. [3] and Aliouane et al. [4] have researched non-destructive testing based on a combination of electromagnetic and sound waves using electromagnetic acoustic transducers, best known as EMATs.

The principle of the eddy current technique is based on the interaction between a magnetic field source and the test material. This interaction induces eddy currents in the test piece [1]. Scientists can detect the presence of very small cracks by monitoring changes in the eddy current flow [5].

Eddy current testing permits crack detection in a large variety of conductive materials, either ferromagnetic or non-ferromagnetic, whereas other non-destructive techniques such as the magnetic particle method are limited to ferromagnetic metals. Another advantage of the eddy current method over other techniques is that inspection can be implemented without any direct physical contact between the sensor and the inspected piece.

(a) Normalized impedance plane. Lift-off curves and crack displacement at impedance plane for two values of conductivity P1 and P2 (adapted from [12]). (b) Altered eddy current flow by a crack on the surface.

When a crack is present in the test piece, it obstructs the eddy current flow, as Figure 2(b) illustrates. There is a displacement from P1 or P2. This causes the eddy current path to become longer, and the secondary magnetic field from the eddy currents is reduced. In conclusion, the real part of impedance Rcn+crack, which is related to eddy current dissipation, decreases Rcn > Rcn+crack, In addition to that, the sum of the primary magnetic field and secondary magnetic field increases, which means that the inductive part of impedance Xcn+crack increases Xcn

Highly conductive materials such as cooper and aluminum create intense eddy currents and have two advantages over less conductive materials. First, cracks generate higher signal levels, as the impedance plane in Figure 2(a) illustrates. In addition to that, the phase lag between the flaws and lift-off line is larger when highly conductive materials are tested, that is φ1 > φ2 as Figure 2(a) shows. The disadvantage of highly conductive materials is that the standard penetration depth is lower at a fixed frequency than in lower conductive materials such as steel and stainless steel. Factors that exert an influence in conductivity are the temperature of the test piece, the alloy composition and the residual stress, which is related to the atomic structure.

The disadvantage of inspecting magnetic materials is that permeability changes generally have a much greater effect on eddy current response than conductivity variations. This heterogeneity means that crack detection is not possible when permeability changes randomly. The equalization of the permeability is often related to how the test piece was manufactured [28]. The heterogeneity of permeability for cast iron is stronger than that of carbon steel [28].

There are methods for lift-off compensation when eddy currents are used in order to detect cracks and lift-off becomes an undesired variable. For instance, Yin et al. researched dual excitation frequencies and coil design to minimize the lift-off effect [32]. Research into processing data is also conducted, with a view to minimizing the lift-off effect. Lopez et al. proposed the use of wavelets to remove eddy current probe wobble noise from steam generator tubes [35]. Reduction of the lift-off effect has also been attempted by optimizing the coil design [36] and sensor array.

In contrast to the conventional eddy-current instrument, pulsed instruments generate square, triangular or a saw tooth waveform [44]. These waveforms have a broad spectrum of frequencies; hence, pulsed eddy current testing techniques provide more information than traditional eddy current testing methods that can be used for the detection and characterization of hidden corrosion and cracking [45]. The data at different frequencies can be correlated to obtain the defect depth.

Pulsed eddy current is useful for more than just crack detection. Haan et al. have used pulsed eddy current to accurately characterize the permeability and the conductivity [48, 49]. Taking a reference measurement of an object with a known thickness, they also determined the thickness of several types of carbon steel materials, which was proportional to the product of conductivity and magnetic permeability.

Some authors have conducted research into pulsed eddy-current techniques. Many years ago, in 1969, Waidelich et al. researched the attenuation of a pulsed field by a conducting sheet [52]. They investigated how to increase the spatial resolution by putting the coil probe in a copper enclosure with a small aperture. Other authors such as Guang et al. presented a system for the inspection of aircraft structures [43]. The system generated pulse excitation that energized a planar multi-line coil of Figure 15(a). The transient field was detected via a giant magnetoresistive GMR field sensor placed on the line of symmetry at the center of the source coil. In the absence of discontinuities, the normal component of the magnetic field was zero at the center of the source coil. When the uniform distribution of the induced currents was distorted by a rivet and/or crack as sketched qualitatively in Figure 15(b) the zero field on the line of symmetry was destroyed and a nonzero transient signal of the normal component was measured by the GMR sensor.

(a) Schematic of the multi-line coil for inducing linear eddy currents (adapted from [43]). (b) Induced eddy current flow in the absence and presence of rivet and cracked rivet (adapted from [43]).

Coil size is also crucial in order to obtain a high-level signal for crack detection. It is crucial that the fill-factor is close to one in the case of encircling coil probes, and it is also crucial that the coil size is similar to the crack size. Some authors such as Grimberg et al. [54] take the coil size into account.

These types of sensors are used in flat surface inspection. The eddy currents on the test piece are circumferences parallel to the surface as Figure 18(a) illustrates. When a penetrating crack occurs on the surface, current flow is strongly altered and the crack can be detected. Pancake-type coil probes are not suitable for detecting laminar flaws as currents flow parallel to the surface and they are not strongly distorted.

Segment probes are used for the detection and control of defects in the weld seam of welded pipes [59]. These probes are available with specific windings and can inspect the tube or bar in differential and absolute modes. Both modes can be implemented in the same probe. In differential mode, the sensor is highly sensitive to punctual defects in the weld seam. Differential segment probes present difficulties detecting long defects in the weld seam of tubes and in the absence of a seam. Differential segment probes only detect the beginning and the end of the crack. To compensate for this disadvantage, absolute mode probes are incorporated along with differential ones to detect the presence or absence of weld seams and long cracks.

On the one hand, the forward solution consists in predicting the impedance or voltage of the eddy-current probe coil when the cracked piece is tested by a probe [64]. Some authors have published models for obtaining the forward solution. For instance, Skarlatos et al. presented a model to solve the forward problem in cracked ferromagnetic metal tubes [58]. Others like La et al. proposed a parametric model to estimate the impedance change caused by a flaw using the electromagnetic quasi-static approach [64]. Bowler et al. solved the harmonic functions of the Laplace equation to calculate the impedance change of the excitation coil inspecting aluminum and steel [65].

Some authors such as Jongwoo et al have researched eddy current testing using Hall-effect sensors. They presented a quantitative eddy current evaluation of cracks on austenite stainless steel using a Hall-effect sensor array [69].

Coil probes provide high sensitivity to defects when eddy current flow is drastically changed. This means that encircling coils are optimized for detecting short discontinuities parallel to the axis of the inspected tubes or bars. Differential encircling probes only detect discontinuities when a long crack that is parallel to the major axis enters and leaves the probe.

Modern instruments generate frequencies in the range from kHz to MHz and permit the application of discrete signal processing, such as filtering and numerical demodulation. Many modern instruments include the impedance on XY plotters and also the X and the Y plot vs. time on LCD screens (or computer monitors if they are computer-enabled). Alarm settings on XY plotters permit users to activate programmable outputs that can activate light and sound alarms to alert the operator when cracks are present [75]. Instruments permit automatic scanning which activates automatic mechanisms to sort flawed pieces or activates paint markers. They also offer very high test speeds that can reduce the occurrence of human errors [76]. 2ff7e9595c